(Issue #752) Add user-side /data/interests (JSON), remove site-side /misc/interestdata

Add a JSON API endpoint for retrieving user interests, and remove the old script.

No replacement is being made for the previous ?int= functionality to find the count for a single interest; this was apparently rarely used.

The new file is based on the Edges.pm file, so I've left the credits for that file as I'm not sure how much of the stuff I deleted is theirs. (Commit b879927 shows the exact amount that was deleted.)

Fixes #752.
Hmm, for the status error codes, I recommend using the methods in DW::Request::Apache2 instead of numbers, for immediate understanding. (I admit of the ones here I'm only familiar with 404 and 403 off the top of my head!)